High spray efficiency and quality of coating applications is important for many industries and for numerous reasons. For example, in the realm of agricultural production, poor quality agrochemical coverage of crops equates to increased pest damage and yield losses. Additionally, off-target movement of pesticides due to drift or runoff may cause environmental pollution of ground water, surface water or air. This could result in pesticide poisonings and other unintended ecological and economic harm. Inadequate mass transfer also wastes significant quantities of chemicals. The end result is that the agricultural producer's financial bottom line suffers due to unnecessarily high costs of chemicals, fuel, equipment and labor. An example from the realm of public health revolves around the effectiveness of spray disinfection and sanitation procedures. Insufficient coverage of human contact surfaces by standard spray equipment has contributed to the large increase in hospital acquired infections such as Methicillin Resistant Staphylococcus aureus (MRSA) and Clostridium difficile as well as the spread of communicable infectious diseases on a wide scale such as SARS, Norovirus, influenza and tuberculosis to name just a few. The cost in such circumstances is great financially as well as in human suffering and lives.
Traditional hydraulic spraying technologies are notoriously inefficient in the mass transfer of sprayed material onto an intended target. Additionally, the material that impacts the target often provides only spotty coverage. Typically, surfaces which are in the direct line of the spray stream or cloud receive a majority of the spray and obscured areas such as the backs and undersides of the target receive almost no coverage. Spray material which doesn't adhere to the target often runs off onto the ground or persists as small droplets in the air which can be easily carried off-site or be inhaled by people in the vicinity of the spray event.
Standard art hydraulic sprayers generally work on the principle of providing high pressure liquid and air flows to a plurality of mechanically atomizing nozzles. The spray of droplets emitting from the nozzles convey relatively high volumes of liquids to an intended target. Inefficiencies in the range of 16-50% stem from the basic inertial, aerodynamic and physical forces of the spray stream. Nozzles on these hydraulic sprayers generally produce large droplets, typically in the range of 100-600 microns in diameter. Such large droplets lead to variable and uneven coverage on target surface. Large droplets are also much more likely to bounce off of or run off of a target and fall to the ground. Agricultural research has shown that traditional hydraulic sprayers apply significantly less material to undersides of leaves and interior portions of target plants than to the adaxial portions of leaves growing on the exterior of the plant canopy. Techniques such as air-assist, droplet size reduction, and electrostatic charging of droplets have all been implemented with varying levels of success. Most promising, so far, is the use of air assisted electrostatic sprayers which operate under low air and liquid pressures and impart a consistently high charge to mass level to the droplets of the uniform spray cloud produced.
Though there have been advances in spray coating technology, several unique circumstances have emerged over the past decade in the fields of agriculture, food safety and public health which demand further enhancements and modifications of current art sprayers to meet the challenges. These novel circumstances bring into play factors which range widely across environmental, economic, political and financial considerations and further impact the nature of how spraying systems are designed.
Agricultural production around the world is trending toward high density plantings to meet growing global population food needs and reduce farm inputs maximize the carrying capacity of the available arable land. High density cultivation with its multiple conservation benefits as well as allowing for higher yields in a smaller area creates challenges for traditional industrial farming implements due to narrow drive rows and dense plant canopies that can be difficult to penetrate with agricultural chemical sprays (Connelly, et al., 2000; Rigg, 1997). Another trend that is seeing a marked increase is the movement of food production indoors or into covered structures. These high density, protected environmental growing conditions hedge against climate instability and extend the production seasons as well as conserving resources such as water and fertilizers. Many of these commercial covered food production systems are even showing up in and near cities and even on building roofs. Again, this poses logistical and cultural challenges for traditional industrial agricultural equipment. Large, loud, heavy diesel tractors blow exhaust and pose chemical drift or runoff hazards which would be unacceptable in such environs. The size and relative lack of maneuverability of such equipment would also be limited within the confines of covered, high-density agriculture.
Just as the manner and locations in which plants and animals are grown for food have changed, so have the chemical inputs to sustain high production levels. Environmental regulations are increasingly reducing the number and type of agrochemical inputs available. Many chemicals are now significantly more expensive than in the recent past. Pest resistances to what chemicals remain further reduce a producer's chemical toolkit. Organic inputs have risen in popularity for their perceived reduced negative impacts on human health and the environment. Since organic products chiefly work only by direct contact with a pest, thorough and even coverage is essential for cost effective control.
An extremely active area of growth in the field of agricultural management is the development and use of biological and biologically-based pesticides known as biorationals. Significant driving forces behind the rising interest include recent regulatory initiatives to deregister numerous chemical pesticides within the next 10 years in many countries. Increasing concern with pesticide residues in food and public spaces has also contributed (Hynes 2006).
Biorational pesticides and biopesticides are derived from a variety of biological sources, including bacteria, viruses, fungi, nematodes and protozoa, as well as chemical analogues of naturally occurring biochemicals such as pheromones and insect growth regulators (IGRs). Applications of live nematodes and insect eggs as biological control methods are also increasing in use. Popularity has been rising for these products and methods which pose little or no adverse environmental or human health effects when used as pest control solutions. Due to a multitude of regulatory factors biorational pesticides are likely to become important factors in agricultural pest management. (Brandenburg, 1999; Guillebeau, 1998)
Many, so called, biorational control products are widely and commercially available. Due to the delicate nature of the organisms, the expense of producing the formulated products, and the need for highly accurate placement of the spray, particular consideration must be given to efficiently and properly apply them. Damaging or otherwise rendering the product or organism non-viable during the application process is of primary concern. It is widely agreed that effective application technologies are required which take into consideration the unique limitations and application parameters of biorational products (Gan-Mor, 2003). Limitations in availability of effective spray application technology currently slow widespread adoption as existing spray technologies are inappropriate in many instances. Handling considerations for biological and biorationals include volume, agitation, pressure and recycling time, system environmental conditions and spray nozzle shear forces, rates and distribution patterns. Research which describes the negative impacts of traditional spray equipment on some of these products is described below.
Beneficial nematodes: Nematode viability has been shown to be negatively impacted by sprayers which pump at pressures greater than 200 kPa. Long pumping periods in high pressure sprayer systems also decreased nematode viability due to the rise in temperature in the liquid after multiple passes through the pump as well as mechanical stresses from piston pumps and the nozzle (Nilsson and Gripwall, 1999). Hydrodynamic damage from fan nozzles is known to damage entomopathogenic nematodes (Fife et al. 2003, 2005). Though three common pumps (centrifugal, diaphragm and roller), when tested, showed no mechanical damage to nematodes after a single passage through each pump at operating pressures up to 828 kPa (120 psi), repeated passages through the pump, such as would be likely for high volume sprayers running at high pressures, caused significant mortality as a result of liquid temperature increases (Klein and Georgis, 1992). Improved control of dosing and delivery to the target site has also been noted as a critical factor for successful use of beneficial nematodes (Shapiro-Ilan, 2006).
Biopesticides: Few biopesticides are currently used commercially as alternatives to chemical pesticides. Part of the problem is due to the lack effective application technologies available to farmers. The success of using existing spray technologies has been very limited due to the inappropriateness of the equipment and complex formulations that would help biopesticides successfully withstand the spraying process. High pressure recirculating pumps have been shown to damage cells and reduce viability. Due to the expense of biopesticides, wastage needs to be minimized. Proper control of droplet spectrum and target deposition specificity also factor greatly into effective utilization of these environmentally benign control products. Non-spore forming bacteria, fungi and viruses are the next generation of pest control products that will lead to improved crop productivity, but also have increased sensitivity to the forces inherent in the spraying process (Hynes 2006). Cells in these formulations are unlikely to perform well in systems with operating pressures higher than 200 MPa or systems with large shear or hydrodynamic forces (Malone, 2002).
Electrostatic spraying equipment employing air-assistance and that can apply the materials with high uniformity and meet delicacy demands of the products is seen as the most promising method of delivery (Gan-Mor, 2003). Research has shown that a sprayer meeting these conditions provided better control when applying a bacterial agent than a standard spray methodology due to the high mass transfer and concentrated nature of the low volume application (Perez et al., 1995).
Perhaps the most widely recognized facet of the rapidly changing world agricultural system is the recent significant decline in honeybee populations. This situation which threatens world food security has given rise to an urgent need for alternative methods of pollinating crops that is cost effective. Artificial pollination using mechanical methods and hand labor currently exist and have been used to a small degree over the years. Proper implementation of these methodologies can lead to significant yield and quality increases. However, high costs and complex, specialized techniques for gathering, storing and applying pollen have limited the use of this practice to only very high value crops (Zhang, 2011; Gan-Mor, 2009, Yi, 2003). Current precision application techniques which consume minimal amounts of the expensive pollen are often extremely labor intensive. Electrostatic application of pollen has shown exceptional promise due to low pollen use and high mass transfer efficiencies (Gan-Mor, 2009; Yi, 2006; George, 2006). However, sufficient particle charges and air velocity are critical factors in successful and economically feasible implementation (Gan-Mor, 2003; Gan-Mor, 2009). The equipment must also be designed such that it does not damage the pollen during application.
Once food is produced, whether it is in the form of leafy greens or meat, there are yet many more points in the journey from field to table that require highly efficient and evolved spray coating technologies. The Food Safety Modernization Act signed into law in 2011 shifted the burden of food safety from the standard of ‘post-incident response’ to the active prevention of food safety hazards and events by producers, packers, shippers and sellers. Recent cases in which food packers and producers have been heavily fined or, in some instances, jailed have driven the need for better, yet still cost effective application technologies and chemicals, some of which can be quite expensive. Bio-rational controls, similar in nature to the ones described above are also under consideration. The ideal system would apply a light, even coating to all surfaces of the target with a high efficiency of mass transfer to minimize waste and maximize protection. Induction charging, air assisted electrostatic application equipment has shown in multiple independent studies and in multiple commodities to perform the needed control in a cost effective manner (Law, 2001).
Public health, aside from food safety scares, is another area that demands improved spray coating equipment and methods. Hospital disinfection and sanitization, prevention of disease transmission in high public use areas as well as the need for bio-terrorism remediation methods, just to name a few, are situations where performance above what is available from standard art hydraulic spraying equipment could provide significant benefits. Again, these are scenarios that require thorough, even and efficient coverage with proper chemicals to prevent or remediate disease causing organisms in a quick response fashion. Hospital Acquired Infections claim millions of lives and even more millions of dollars per year. Many if not most of these HAI's are preventable with proper sanitation.
Research was performed in 2013 at an assisted care facility in VA using a high efficiency, air-assisted electrostatic sprayer to spray disinfectants to patients rooms three times per week. Results showed that this method, when compared to standard cleaning methods, saved significant time, material and labor and left ‘high touch’ areas with far fewer bacteria (http://pacificcrestpa.com/wp-content/uploads/2013/07/Clinical-Trial-Outcomes-MFA-6-12-13-copy.pdf). This method of sanitizing also was able to control a difficult C. difficile outbreak in the facility (http://www.haisolutionsllc.com/images/pdfs/steriplex-sd-testimony.pdf). As global travel and population interactions continue to increase, epidemics of communicable diseases such as Avian and Swine flu, SARS or MERS as well as influenzas will become more common and will require rapid response to contain the spread.
At the current time there is no one piece of equipment that could serve the many and varied needs, specifications and requirements of all of the potential spray coating applications and scenarios described above. Therefore, there is a need in the art for a method and apparatus for the highly effective and efficient spray application of liquid suspensions containing delicate microscopic particles such as biorationals, pollen or, as-yet-to-be-developed, specialized nano-materials to three dimensional surfaces in addition to traditionally sprayed materials. Additional requirements of such apparatus are a relatively small size and weight factor but with sufficient power to provide the requisite aerodynamic and electrical forces for maximum liquid mass transfer and surface coverage.